Lithography is the process of printing from specially prepared surfaces, some areas of which are capable of accepting lithographic ink, whereas other areas, when moistened with water, will not accept the ink. The areas which accept ink form the printing image areas and the ink-rejecting areas form the background areas.
In the art of photolithography, a photographic material is made imagewise receptive to oily or greasy ink in the photo-exposed (negative working) or in the non-exposed areas (positive working) on a hydrophilic background.
In the production of common lithographic plates, also called surface litho plates or planographic printing plates, a support that has affinity to water or obtains such affinity by chemical treatment is coated with a thin layer of a photosensitive composition. Coatings for that purpose include light-sensitive polymer layers containing diazo compounds, dichromate-sensitized hydrophilic colloids and a large variety of synthetic photopolymers. Particularly diazo-sensitized systems are widely used.
Upon imagewise exposure of such light-sensitive layer the exposed image areas become insoluble and the unexposed areas remain soluble. The plate is then developed with a suitable liquid to remove the diazonium salt or diazo resin in the unexposed areas.
Imaging elements which comprise a photosensitive composition are called photo mode imaging elements
On the other hand, heat mode imaging elements, the surface of which can be made image-wise receptive or repellant to ink upon image-wise exposure to heat obtained by conversion of irradiation into heat and in most cases a subsequent development are also known for preparing lithographic printing plates. A particular disadvantage of photo mode imaging elements such as described above for making a printing plate is that they have to be shielded from the light. Furthermore they have a problem of sensitivity in view of the storage stability and they show a lower resolution. The trend towards heat mode printing plate precursors is clearly seen in the market.
Interesting heat mode imaging materials are those that have as heat mode layer a layer capable of being ablated by actinic radiation as described in e.g. U.S. Pat. Nos. 5,379,698, 5,353,705, EP-A-683,728, 678,380, 649,374, 580,393, 580,394 and DE 2,512,038. Particularly interesting heat sensitive imaging elements are those where said layer capable of being ablated by actinic radiation is a layer of a low melting, non-toxic metal with a low thermal conductivity metal such as aluminum, bismuth, tin, indium, tellurium etc.
A problem that arises with the present ablation based printing plate precursors is that said ablation process can cause formation of debris originating from the ablatable layer itself or of other functional layers of said precursor. Said debris can interfere with transmission of the laser beam (e.g. by depositing on a focusing lens or as an aerosol that partially blocks transmission) or with the transport of the imaging element during or after recording when this debris remains loosely adhered to the plate and deposition of said debris occurs on the transport rollers. So, there is a need for heat mode imaging elements based on ablatable layers which upon actinic radiation do not lead to said debris related problems.